The present disclosure relates to single-longitudinal mode laser devices.
Single-longitudinal mode (SLM) laser is an essential laser device in a wide range of applications from stable operation of intra-cavity frequency doubling, precision measurement, high-resolution spectroscopy, coherent lidars, coherent optical communication, to laser trapping or cooling. As already known, in the common standing-wave lasers, and especially homogeneously broadened solid-state lasers, spatial hole-burning in active gain materials usually causes multimode laser operation. Various techniques have been attempted to obtain SLM laser operation.
One conventional SLM laser includes twisted mode in the laser cavity. Referring to FIG. 1A, a conventional SLM laser 100 can include a pair of mirrors 111, 112 that define a laser cavity 120, a linear polarizer 130, a gain medium 140, a quarter wave plate 150, and a quarter wave plate 160 in the laser cavity 120. A pump light 170 is transmitted into the laser cavity 120. A lasing light 180 is generated by the gain medium 140 in response to the pump light 170. The mirror 111 is totally reflective to the lasing light 180 while the mirror 112 is semi reflective and semi-transmissive. The lasing light 180 is reflected by the mirrors 111, 112 to establish a standing wave in the laser cavity. In the forward pass as shown in FIG. 1B, the linear polarizer 130 polarizes the lasing light 180 in a linear direction (P1). The quarter wave plate 150 transfers the linearly polarized lasing light (P1) to a circularly polarized lasing light (P2) that enters the gain medium 140. The gain medium is required to be isotropic, which can maintain the circular polarization (P2) along the forward pass. The quarter wave plate 160 returns the circular polarization (P2) into a linear polarization (P3). The linear polarization (P4) is maintained after the reflection by the mirror 112. The quarter wave plate 160 produces another circularly polarized light with a circular polarization (P5) that is opposite to the circular polarization P2 in the forward direction. The quarter wave plate 150 then returns circular polarization (P5) to another linearly polarized (P6) lasing light, which is parallel to the polarization axis of the linear polarizer 130. The linear polarization (P6) is maintained after the lasing light is reflected by the mirror 111 and passes the linear polarizer 130 in the next forward pass. A portion of the lasing light 180 can transmit through the mirror 112 to form an output laser light 185.
Because the gain medium 140 is isotropic, the circularly polarized lasing light 180 in the forward and the backward direction have the same intensity as well as the same frequency. The oppositely circularly polarized lasing light 180 in the forward and the backward directions can prevent the formation of a standing wave, which suppresses hole burning effect and enables a single longitudinal mode in the laser cavity 120. A limitation of the SLM laser 100, however, is that the gain medium is required to be an isotropic material in order to produce a single longitudinal mode.
In another attempt, a thin gain medium crystal is positioned close to one of the mirrors in a laser cavity to produce single longitudinal mode. Since all modes have a common node at the mirror and to a large extent share the same population of ions in the vicinity of the mirror, the effect of spatial hole burning can therefore suppressed. A gain medium with short absorption depth can also be used instead of a thin gain-medium crystal. This type of lasers is expected to operate at the single-longitudinal mode when the pump power of laser diode is less than 5 times the thresholds. This approach is, however, incapable of producing high-power output r due to the size limitation of the thin gain medium. Moreover, cavity length must be selected by temperature control so that one of its resonant frequency fall within the laser gain region.
Laser operation in a ring cavity is another known technique to obtain SLM output. In this configuration, an intra-cavity optical diode keeps unidirectional laser propagation so that no standing-wave electric fields are formed in the cavity, leading to the elimination of spatial hole-burning in the active material. But this method cannot be used for microchip laser.